I agree on trying to look in the radiator. At that age, you’re asking for trouble installing and removing hoses if they’ve never been removed.
Personally, if mine, I’d do a drain and fill with the correct Toyota coolant like yesterday. Then figure it out from there.
That may mean some cleaning fluid in there depending upon how clean it is or isn’t. I’d suspect you’ll find it’s very clean in there.
My 96 Ram has its original radiator after 438k. But it’s pretty fouled. It works perfect under full rated payload and long sustained climbs in my area, and I’m hesitant to do any scale remover given that it works.
The yellow coolant in there is JD Cool Gard II, but the fouling is because it’s a brass radiator and cast iron engine... but the fouling I think is due to ages with green...
I blame green... this is my 1981 MB 240D with all original cooling system. It has had G05 its whole life.
And here’s my 98 S-10, I’ve owned since new and it’s had Dexcool since new.
I guess we didn’t get the memo that Dex should be sludged up and ruin everything. Especially in this old school system design.
So, point being, I suspect that your system is clean and quite fine, assuming no other coolants were mixed in or other issues have arisen? So the key is to get some new chemistry in there to replenish any depleted chemistry, and the reason to get some fresh in ASAP is to help replenish anything that has a slow reaction to passivate surfaces. Assuming your system works fine, it will buy you time until warmer weather to do any flushes if need be - do it in the spring with multiple courses of distilled water flushing if needed. If no special chemical flush is needed, then leave it, and do another drain and fill with more Toyota coolant in the spring, maybe two or three times to replenish more chemistry, and then keep rolling.
